So-called “variable orifice nozzles” (VONs) enable the number of orifices that are used to inject fuel into a combustion space to be varied for different engine loads. Typically, such a nozzle includes a nozzle body which is provided with a blind bore within which a first, outer valve needle is moveable under the control of an actuator. The nozzle body bore defines a seating surface with which the outer valve needle is engageable to control fuel injection through a first set of nozzle outlets provided at a first axial position in the wall of the nozzle body. The outer valve needle itself is provided with a longitudinally extending bore opening at the valve tip and within which a second, inner valve needle is moveable. The inner valve needle projects from the opening of the outer valve needle and is engageable with the seating surface to control fuel injection through a second set of outlets provided at a lower axial position in the wall of the nozzle body than that of the first set of nozzle outlets.
In a known injection nozzle of this type, as described in the Applicant's co-pending European patent application no. EP 04250928.1, the fuel flow to a first (upper) set of nozzle outlets is controlled by an outer valve needle and the fuel flow to a second (lower) set of nozzle outlets is controlled by an inner valve needle. In order to deliver fuel through the upper outlets, the outer valve needle alone is operable to disengage its seating but the inner valve needle remains seated. In order to deliver fuel through the lower outlets in addition to the upper outlets, the outer valve needle is permitted to move beyond a pre-determined distance such that its movement is transmitted to the inner valve needle causing the inner valve needle to disengage or lift from its seating also. During this latter stage of operation, both the first and second sets of outlets are opened to provide a relatively high fuel delivery rate. An injection nozzle of this type enables selection of a small total nozzle outlet area in order to optimise engine emissions at relatively low engine loads. Alternatively, a large total nozzle outlet area may be selected so as to increase the total fuel flow at relatively high engine loads.
In the above described injection nozzle, positional control of the outer valve needle is typically achieved through the use of a piezoelectric stack-type actuator, the movement of which is transmitted to the outer valve needle by way of a direct mechanical or hydraulic coupling. A piezoelectric actuator is particularly suitable to this type of injection nozzle since it is energy efficient and enables precise control of valve needle lift. However, piezoelectric actuators are expensive to manufacture so there is a need to retain the benefits of variable orifice nozzles whilst utilising less expensive means of controlling injection.